Finding enum value with Java 8 Stream API

Suppose there is a simple enum called Type defined like this:

enum Type{
    X("S1"),
    Y("S2");

    private String s;

    private Type(String s) {
        this.s = s;
    }
}

Finding the correct enum for given s is trivially done with static method with for-loop (assume the method is defined inside enum), e.g.:

private static Type find(String val) {
        for (Type e : Type.values()) {
            if (e.s.equals(val))
                return e;
        }
        throw new IllegalStateException(String.format("Unsupported type %s.", val));
}

I think the functional equivalent of this expressed with Stream API would be something like this:

private static Type find(String val) {
     return Arrays.stream(Type.values())
            .filter(e -> e.s.equals(val))
            .reduce((t1, t2) -> t1)
            .orElseThrow(() -> {throw new IllegalStateException(String.format("Unsupported type %s.", val));});
}

How could we write this better and simpler? This code feels coerced and not very clear. The reduce() especially seems clunky and abused as it doesn't accumulate anything, performs no calculation and always simply returns t1 (provided the filter returns one value - if it doesn't that's clearly a disaster), not to mention t2 is there superfluous and confusing. Yet I couldn't find anything in Stream API that simply somehow returns directly a T from a Stream<T>.

Is there a better way?

Answers


I would use findFirst instead:

return Arrays.stream(Type.values())
            .filter(e -> e.s.equals(val))
            .findFirst()
            .orElseThrow(() -> new IllegalStateException(String.format("Unsupported type %s.", val)));


Though a Map could be better in this case:

enum Type{
    X("S1"),
    Y("S2");

    private static class Holder {
        static Map<String, Type> MAP = new HashMap<>();
    }

    private Type(String s) {
        Holder.MAP.put(s, this);
    }

    public static Type find(String val) {
        Type t = Holder.MAP.get(val);
        if(t == null) {
            throw new IllegalStateException(String.format("Unsupported type %s.", val));
        }
        return t;
    }
}

I learnt this trick from this answer. Basically the class loader initializes the static classes before the enum class, which allows you to fill the Map in the enum constructor itself. Very handy !

Hope it helps ! :)


The accepted answer works well, but if you want to avoid creating a new stream with a temporary array you could use EnumSet.allOf().

EnumSet.allOf(Type.class)
       .stream()
       .filter(e -> e.s.equals(val))
       .findFirst()
       .orElseThrow(String.format("Unsupported type %s.", val));

Arrays.stream(Type.values()).filter(v -> v.s.equals(val)).findAny().orElseThrow(...);

How about using findAny() instead of reduce?

private static Type find(String val) {
   return Arrays.stream(Type.values())
        .filter(e -> e.s.equals(val))
        .findAny()
        .orElseThrow(() -> new IllegalStateException(String.format("Unsupported type %s.", val)));
}

I think the second answer of Alexis C. (Alexis C.'s answer) is the good one in term of complexity. Instead of searching in O(n) each time you look for a code using

return Arrays.stream(Type.values())
        .filter(e -> e.s.equals(val))
        .findFirst()
        .orElseThrow(() -> new IllegalStateException(String.format("Unsupported type %s.", val)));

you could use O(n) time at the loading of the class by putting all elements into the map, and then access to the code of the type in constant time O(1) using the map.

enum Type{
X("S1"),
Y("S2");

private final String code;
private static Map<String, Type> mapping = new HashMap<>();

static {
    Arrays.stream(Type.values()).forEach(type-> mapping.put(type.getCode(), type));
}

Type(String code) {
    this.code = code;
}

public String getCode() {
    return code;
}

public static Type forCode(final String code) {
    return mapping.get(code);
}
}

I know this question is old but I came here from a duplicate. My answer is not strictly answering the OP's question about how to solve the problem using Java Streams. Instead, this answer expands the Map-based solution proposed in the accepted answer to become more (IMHO) manageable.

So here it is: I propose to introduce a special helper class that I named EnumLookup.

Assuming the Type enumeration is slightly better written (meaningful field name + getter), I inject an EnumLookup constant to it like below:

enum Type {

    X("S1"),
    Y("S2");

    private static final EnumLookup<Type, String> BY_CODE = EnumLookup.of(Type.class, Type::getCode, "code");

    private final String code;

    Type(String code) {
        this.code = code;
    }

    public String getCode() {
        return code;
    }

    public static EnumLookup<Type, String> byCode() {
        return BY_CODE;
    }
}

The usage then becomes (again, IMO) really readable:

Type type = Type.byCode().get("S1"); // returns Type.X

Optional<Type> optionalType = Type.byCode().find("S2"); // returns Optional(Type.Y)

if (Type.byCode().contains("S3")) { // returns false
    // logic
}

Finally, here's the code of the EnumLookup helper class:

public final class EnumLookup<E extends Enum<E>, ID> {

    private final Class<E> enumClass;
    private final ImmutableMap<ID, E> valueByIdMap;
    private final String idTypeName;

    private EnumLookup(Class<E> enumClass, ImmutableMap<ID, E> valueByIdMap, String idTypeName) {
        this.enumClass = enumClass;
        this.valueByIdMap = valueByIdMap;
        this.idTypeName = idTypeName;
    }

    public boolean contains(ID id) {
        return valueByIdMap.containsKey(id);
    }

    public E get(ID id) {
        E value = valueByIdMap.get(id);
        if (value == null) {
            throw new IllegalArgumentException(String.format(
                    "No such %s with %s: %s", enumClass.getSimpleName(), idTypeName, id
            ));
        }
        return value;
    }

    public Optional<E> find(ID id) {
        return Optional.ofNullable(valueByIdMap.get(id));
    }

    //region CONSTRUCTION
    public static <E extends Enum<E>, ID> EnumLookup<E, ID> of(
            Class<E> enumClass, Function<E, ID> idExtractor, String idTypeName) {
        ImmutableMap<ID, E> valueByIdMap = Arrays.stream(enumClass.getEnumConstants())
                .collect(ImmutableMap.toImmutableMap(idExtractor, Function.identity()));
        return new EnumLookup<>(enumClass, valueByIdMap, idTypeName);
    }

    public static <E extends Enum<E>> EnumLookup<E, String> byName(Class<E> enumClass) {
        return of(enumClass, Enum::name, "enum name");
    }
    //endregion
}

Note that:

  1. I used Guava's ImmutableMap here, but a regular HashMap or LinkedHashMap can be used instead.

  2. If you mind the lack of lazy initialization in the above approach, you can delay building of the EnumLookup until byCode method is first called (e.g. using the lazy-holder idiom, like in the accepted answer)


I can't add a comment yet, so I am posting an answer to complement the above answer, just following the same idea but using java 8 approach:

public static Type find(String val) {
    return Optional
            .ofNullable(Holder.MAP.get(val))
            .orElseThrow(() -> new IllegalStateException(String.format("Unsupported type %s.", val)));
}

You need a getter for String s. In the example below this method is getDesc():

public static StatusManifestoType getFromValue(String value) {
    return Arrays.asList(values()).stream().filter(t -> t.getDesc().equals(value)).findAny().orElse(null);
}

You'd need a getter for String s, but this is the pattern I use:

private static final Map<String, Type> TYPE_MAP = 
    Collections.unmodifiableMap(
        EnumSet.allOf(Type.class)
        .stream()
        .collect(Collectors.toMap(Type::getS, e -> e)));

public static Type find(String s) {
    return TYPE_MAP.get(s);
}

No for loops, only streams. Quick lookup as opposed to building a stream every time the method is called.


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